It is frequently necessary to perform leak testing of small sealed parts. An example is the impact sensor used in the airbag system of an automobile. The impact sensor includes an impact switch that is activated by the deceleration of a collision. The impact switch activates an airbag inflation device. The impact switch is enclosed in a hermetically sealed container to prevent contamination by moisture and other foreign matter. Since proper operation of the impact sensor is critical to the function of the airbag system, a leak test must be performed on the impact sensor to ensure seal integrity. The leak test must be capable of detecting leaks over a wide range of leak rates.
One prior art technique for leak testing small parts, such as impact sensors, involves immersion of the part in a fluid such as a hydrofluorine compound that has a low boiling point. Parts with leaks fill up with the fluid. Then the parts are immersed in a heated fluid which causes the hydrofluorine compound to boil, thereby creating bubbles when there is a leak. A human operator must observe the bubbles in order to detect leaks. The bubble leak detection technique is an extremely slow, unreliable and expensive process. Furthermore, the bubble technique is unable to detect fine leaks.
Another prior art technique for detecting gross leaks involves measurement of the time required for vacuum pumping. A part under test is placed in a test fixture, and the test fixture is vacuum pumped. The time required to reduce the test fixture to a prescribed pressure is measured. When the test part has a gross leak, the time to reach the prescribed pressure increases, thereby indicating a leaky part.
Yet another prior art leak detection technique involves the use of a helium mass spectrometer leak detector. Helium is used as a trace gas which passes through the smallest of leaks in a sealed test part. The helium is the n drawn into a leak detection instrument and is measured. An important component of the instrument is a mass spectrometer tube which detects and measures the helium. The input test sample is ionized and mass analyzed by the spectrometer tube in order to separate the helium component. For testing of small parts, such as an impact sensor, the test part is pressurized with helium and is placed in a sealed test fixture connected to the test port of the leak detector. The helium, which passes through leaks in the test part into the test fixture, is drawn through the test port into the leak detector and is measured by the leak detector. Helium mass spectrometer leak detectors are disclosed, for example, in U.S. Pat. No. 3,690,151 issued Sep. 12, 1972 to Briggs; U.S. Pat. No. 4,499,752 issued Feb. 19, 1985 to Fruzzetti et al and U.S. Pat. No. 4,845,360 issued Jul. 4, 1989 to Landfors.
While helium mass spectrometer leak detectors provide highly satisfactory operation under appropriate circumstances, gross leaks can not be reliably detected in sealed parts such as impact sensors, because the part may have a gross leak, and all helium may have leaked from the test part prior to the time of leak testing. In this case, the helium mass spectrometer leak detector misinterprets a gross leak as no leak because no helium is detected.
It is a general object of the present invention to provide improved leak detection techniques over the prior art.
It is another object of the present invention to provide methods and apparatus for reliably detecting gross leaks as a pre-test to fine leak detection.
It is a further object of the present invention to provide methods and apparatus for detecting leaks over a wide range of leak rates.
It is yet another object of the present invention to provide leak detection methods and apparatus which are fast, easy to use and highly reliable.